Quote:
Originally Posted by stoatwblr
They don't. In fact they (and single fat pipes) can seriously damage economy. The only benefit they give is at maxpower/rpm (Wide open throttle)
Otto engines need back pressure at low speeds or they run badly/stall. You'e be better off working out what rpms the engine runs and and tuning the extract pipes so the valve opening impulse is reflected back to the exhaust port out of phase, for the next opening at the most common speed (It provides a suction effect). That's what tuned exhausts on 2 strokes do.
You can tune for multiple engine speeds by using paralleled varying lengths, but space and weight become issues.
As the Prius uses Miller cycle (effectively a short stroked inlet stroke via extremely late inlet valve closure), even that might badly affect the economy. This is the kind of thing which requires decent computational power and a lot of dyno testing to verify any changes.
I'm a little surprised that Toyota isn't taking an interest in what hypermilers do in order to roll changes that work into their halo cars.
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Not true ever and at all. Back pressure is a myth. Ideally you want a properly sized exhaust so that the flow(speed too) of the gases will assist in pulling exhaust through the system. I agree with your explanation, but back pressure is a no go, there may be pressure build up(i know there is) but it's not a desired effect.